Hitherto, as a method for joining (bonding) a ceramic member and a metal circuit member, a simultaneous sintering method (co-fire method) of printing paste of a high melting point metal (refractory metal) such as Mo or W on the surface of a ceramic sheet-shaped molding and then sintering, a DBC method (direct bonding copper method) in which a circuit layer is integrally joined on the surface of a ceramic substrate using a eutectic reaction between copper serving as a circuit material and oxygen, an active metal brazing method using a brazing material containing an active metal such as Ti as a joining material of a metal circuit layer, and the like have been widely used.
Joined bodies of a ceramic member and a metal member produced by the above-described joining methods are used in various fields. A typical example thereof is a ceramic circuit board for mounting and joining a semiconductor device and the like. Examples of characteristics required for such a ceramic circuit board include satisfactory heat dissipation effect (heat radiating property), high structural strength as a whole ceramic circuit board, high joint strength (bonding strength) between a ceramic substrate and a metal circuit plate, and satisfactory heat resistance cycle characteristics as a circuit board.
As a ceramic substrate constituting the ceramic circuit board, a sintered body of aluminum nitride (AlN), aluminum oxide (Al2O3), or silicon nitride (Si3N4) has been widely used.
For example, an aluminum nitride substrate is particularly excellent in heat dissipation effect because it has a high thermal conductivity of 160 W/m·K or more, which is higher than that of other ceramic substrates. A silicon nitride substrate has a three-point bending strength (at room temperature) of 600 MPa or more, and thus the strength of the circuit board can be improved when silicon nitride is used as a material for a ceramic substrate. In contrast, an aluminum oxide substrate has a thermal conductivity of about 20 W/m·K and a three-point bending strength of about 360 MPa. Therefore, in particular, in order to achieve high heat dissipation effect and high structural strength, use of a nitride ceramic substrate is more preferable for a circuit board, compared with an oxide ceramic substrate.
In view of the joint strength between a ceramic substrate and a metal circuit plate, among the above-described joining methods, the active metal brazing method is preferred. In the active metal brazing method, a metal foil containing at least one active metal such as Ti, Hf, Zr, and Nb or paste prepared by adding these active metals to an Ag—Cu brazing material is applied between the ceramic substrate and the metal circuit plate, and both members are then integrally joined and bonded by heat treatment. When a joining by the active metal brazing method is performed using a nitride ceramic substrate, a joint layer composed of a nitride of the above active metal is formed after the heat treatment, resulting in the formation of a stronger joint state. Thus, a joined body of a nitride ceramic and a metal member prepared by the active metal brazing method satisfies the characteristics required for a circuit board. Accordingly, such a joined body has been widely used as a substrate for electronic circuits, such as a substrate for a semiconductor module (power module) on which a power semiconductor device is mounted.
In addition, as a known insulating circuit board for mounting a semiconductor, the following circuit board has also been proposed: The circuit board has a structure in which, for example, a metal circuit layer is laminated and bonded on at least one of the surfaces of a ceramic substrate with an Al—Si— or Al—Ge-based brazing material for the metal layer therebetween. By adjusting the Vickers hardness and the thickness of the metal layer and the thickness and the flexural strength of the ceramic substrate to predetermined values, the heat resistance cycle life of the circuit board is extended (see, for example, Patent Document 1).
Furthermore, as a known ceramics wiring board, a wiring board prepared by forming a metal layer such as an Al or Ni layer that has excellent wettability with a brazing material and that has a thickness of 1 to 10 μm on the surface of a ceramic substrate has also been proposed (see, for example, Patent Document 2).
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 2001-144234
[Patent Document 2] Japanese Unexamined Patent Application Publication No. 2002-111211
However, in the above known circuit boards, although the structural strength is improved to some degree, the heat resistance cycle characteristics do not necessarily satisfy the present technical requirement. The reason for this is as follows: As the capacity, the output, and the integration degree of recent semiconductor devices increase, the heat generating amount from the devices tends to increase. When the heating value increases, cracks due to the difference in thermal expansion between the metal circuit plate and the ceramic substrate are easily generated in the ceramic substrate and the brazing material layer, resulting in problems such as a decrease in the withstand voltage of the ceramic substrate and separation of the metal circuit layer. In particular, when the metal circuit layer is joined by the active metal brazing method, a phase of a nitride of the active metal is formed on the surface adjacent to the nitride ceramic. Although this active metal nitride phase effectively acts on the improvement of the joint strength, the phase does not have a function for releasing a stress generated by the difference in thermal expansion. Consequently, cracks are easily generated in the ceramic substrate, resulting in the problem of degrading the durability of the circuit board.
In order to solve the above-described problem, a method is also employed in which, instead of the copper plate serving as the circuit layer, an aluminum plate is used and is joined with a ceramic substrate with an Al alloy brazing material therebetween. The aluminum has not only a conductivity next to that of copper and high heat dissipation effect but also a property that plastic deformation is easily caused by a thermal stress. Therefore, the generation of cracks in the ceramic substrate, solder, and the like can be prevented.
On the other hand, an Al—Si alloy forms a joint by bonding to oxygen that is present on the surface of the ceramic substrate. However, unlike a substrate made of an oxide ceramic represented by alumina, particularly in a substrate made of a nitride ceramic such as aluminum nitride or silicon nitride, since the oxygen content per unit area of the substrate structure is low, the bondability (i.e., wettability of the interface between the Al—Si alloy and the ceramic) is low. In order to compensate for this disadvantage, the joining is performed while a load is applied. However, because of a problem relating to the bondability with aluminum nitride, and in particular, silicon nitride, the heat resistance cycle characteristics are varied. Thus, the countermeasure is not sufficient.
As described above, in order to improve the wettability of the brazing material, a method of forming an Al metal film on the surface of a ceramic substrate and then joining the substrate is also employed. However, a phenomenon in which the surface of the structure partly rises because of the diffusion of Al elements in the Al metal film, i.e., the hillock phenomenon, occurs. Consequently, voids (air gaps) are easily formed between the Al metal film and the Al—Si brazing material, resulting in a decrease in the joint strength of the metal circuit layer. Thus, disadvantageously, the heat resistance cycle characteristics as the whole circuit board is easily degraded. Furthermore, since the Al metal film is formed so that the thickness of the film is as large as about 1 to 10 μm, the time required for vapor deposition of Al metal is increased, resulting in the problem of an increase in the production cost.
In the semiconductor field, in addition to the developments of integration degree and the increase in the operation speed of LSIs, for example, the applications of power devices such as GTO and IGBT (insulated gate bipolar transistor) have been increasing. Under this situation, the heating value (amount of heat) generated from a silicon chip (semiconductor element) has been steadily increasing. When the power modules are used in the fields requiring a long-term reliability, such as electric railroad vehicles and electric automobiles, the heat dissipation effect and the durability of a circuit board having a silicon chip thereon or a module including the circuit board therein are of more concern. Unfortunately, in the known power modules, the durability of the joined part is insufficient and satisfactory reliability cannot be ensured.
The present invention has been made in order to solve the above-mentioned problems in the related art. In particular, it is an object of the present invention to provide a ceramic circuit board in which the generation of voids in the joint interface can be effectively suppressed, the joint strength of a metal member serving as a circuit layer can be increased, and the heat resistance cycle characteristics can be drastically improved, a method for producing the same, and a power module including the circuit board.